Fair Isle Renewed
W. M. Somerville M.Sc.
Newcastle upon Tyne

THE WIND TURBINES
 

The design of the new wind turbine and the refurbishment of the old turbine draw heavily on the experience of operating in .the extreme conditions on Fair Isle. The early gearbox failures on the first machine highlighted the futility of putting a brake on the output shaft of the gearbox and the risk, fortunately avoided by prompt action, of a runaway turbine rotor and potential destruction of the entire turbine. The enforced shut down of the old turbine in 1996 was due to the failure of the low speed shaft oil seal on the gearbox. Positioned between the windshaft main bearings, the seals could not be replaced without dismantling the complete turbine and nacelle.

The design, for all such isolated sites, must place a high priority on safety, damage limitation factors and ease of servicing and repair. This philosophy required a completely new nacelle layout for the old turbine, placing the mechanical brake on the wind shaft between the main bearings and the gearbox on an overhung portion of the shaft behind the main bearings. The final drive to the generator is by wedge belts which, although somewhat less efficient, will act as a torque limiter by slipping if seriously overloaded. These wedge belts are not expensive and are easily replaced if worn or damaged. The mechanical brakes are "fail-safe", applied by springs and released by compressed air at 7 bar, provided the control solenoid valve is energised. Thus, loss of control current or loss of air pressure will automatically apply the brakes and stop the turbine. However, the loss of air pressure may be slow and allow the brakes to come on slowly and cause the pads to wear out. To prevent this, a pressure switch will reset if the line presssure falls below that needed to fully release the brakes, and this will, in turn, apply the brakes immediately. The key information on these two, fixed pitch, stall regulated, turbines is listed below:
 

The gearbox and the generator from the old turbine were both inspected and found to be in good condition when the machine was dismantled. These units were overhauled, fitted with new bearings, and reused in the 60kW nacelle. The tower required minor modification to the top flange and was also reused.

Safety and damage limitation considerations led to a number of sensors and switches being installed on each turbine for the following functions:

1. Rotor overspeed.

2. Generator overheat.

3. Main gearbox oil level low.

4. Low brake release air pressure.

5. Cable twist exceeds four turns.

6. Drive belts damaged or broken.

7. Brake adjustment required.

8. Compressor running.

9. Nacelle safety lockout switch.

These functions are self-explanatory and for this site considered essential. Further, the overspeed sensors and relays are duplicated, and the relays set up in a "fail-safe" mode. All these sensors and switches provide indication to assist the operators, both at the local control pedestal at the base of the tower and in the turbine control panel in each control room.

A small pony motor was installed in each nacelle to aid start-up in marginal wind conditions. This, it was hoped, would reduce the time required to commission each turbine when shaft seals and belts were new and stiff. It was also thought to be a wise precaution in the event that the self-starting capability of the blade settings proved to be underestimated. The compressed air to release the brakes is provided by a small commercial, single phase compressor, which is mounted on, and automatically charges, a 25 litre receiver in the nacelle, to 10 bar.

The turbine blades are equipped with "twist tip" air brakes, centrifugally released on overspeed and mechanically linked to trigger at the same time. For simplicity the manually reset option was selected.